The present invention relates to a method and apparatus for molding thermosetting materials and, more particularly, to the molding of thermosetting synthetic resin compositions.
Injection molding is an advantageous method of forming articles of synthetic resin. In general terms, injection molding is a process wherein the molding composition, in a melted or plasticized state, is injected into a mold cavity. Typically, cold molding composition in granular or nodular form is fed from a hopper into a heated cylinder containing a screw. The molding composition is heated, melted and plasticized on the screw flights, and then the screw, acting as a ram, injects the melted and plasticized material into a mold cavity. In the case of thermosetting material, the molded material is cured in the mold by compaction and by heat added to the mold cavity. After curing, the article is ejected from the mold and the process repeated.
Injection molding offers the advantages of reduced molding cycles, better control of process variables, and increased productivity as compared with conventional compression and transfer molding processes. The major disadvantage with the injection molding of thermosetting materials is the generation of a considerable amount of waste material, particularly when multiple cavity systems are employed. The waste material is generated by thermosetting material that has cured, become infusible, in the runner and sprue systems and cannot be reused. The amount of non-reusable waste material generated in this fashion can be substantial, ranging typically from about 15 to about 80 percent of the total amount of material required to mold an article.
A more recent technical advance in the molding art has been the adaptation of the runnerless injection, or cold manifold, process to the injection molding of thermosetting resins. In the cold manifold process, the material in the sprue and manifold system is maintained at a temperature sufficient to maintain the material in a plasticized condition, without causing the material to prematurely cure or "set-up". Thus, when a cured part is removed from the mold cavity, the material in the sprue and manifold becomes part of the next molding, instead of being discarded as in conventional injection molding operations. The runnerless injection process, therefore, provides for significant savings in material.
The thermosetting materials usually employed in runnerless injection processes differ in some respects from materials normally employed in conventional injection processes because of the different requirements of each process. One significant difference is that a standard injection molding material typically has a stiffer plasticity for faster molding cycles. In contrast, a runnerless injection material is adapted to remain in a plasticized or fused condition in the feed system for extended periods of time without prematurely curing, usually at temperatures between about 104.degree. and 116.degree. C. (220.degree. to 240.degree. F.) while also being capable of rapidly curing in the mold cavity at the molding temperature, usually about 170.degree. C. (340.degree. F.). Examples of suitable runnerless injection molding compositions are described in U.S. Ser. No. 915,335, filed June 13, 1978, now abandoned, but refiled as applications Ser. Nos. 030,037 and 030,038, on Apr. 13, 1979. The disclosure in the referenced application is hereby incorporated by reference. Although such formulations are useful in the present invention, they are not required, and the molding compositions presently utilized may be selected from the more economical and more readily available standard thermosetting molding compositions.
Thermosetting molding materials useful in the present invention may suitably be selected from thermosetting synthetic resins and resin compositions typically used in molding operations, for example, phenolic; amino, such as urea, melamine and melamine/phenolic; polyester resins, including unsaturated polyesters, in granular, nodular, bulk or sheet forms; alkyd; epoxy; silicone; diallylphthalate; polyamides, or from thermosetting natural and synthetic rubber compositions. Phenolic resin compositions are especially useful as the feed material. Phenolic resin compositions used in molding operations are usually employed in the form of molding compositions. Phenolic molding compositions typically are particulate in form containing a molding grade phenolic resin, a cross-linking agent such as hexamethylenetetramine, and suitable filler materials.
The technique of injection-compression molding basically consists of injecting a charge of plasticized molding material into a partially open mold; the final fill, or mold fit, is accomplished by the subsequent complete closure of the mold. Injection-compression molding makes possible a combination of the positive attributes of compression molding, i.e., improved dimensional stability, uniform density, low shrinkage and impact strength, with the automation and fast cure of injection molding.
The present invention provides a method and apparatus for adapting the advantages of injection-compression molding to include the advantages of runnerless injection techniques and facilitates the use of standard phenolic molding compositions in such method and apparatus.